1. The Field of the Invention
The present invention relates to a biologically degradable polymer mixture, to a process for its preparation and to a method of processing the polymer mixture according to the invention. More particularly, the invention relates to the manufacture of biodegradable polymer blends that include thermoplastic starch and polymers that are at least partially biodegradable.
2. The Relevant Technology
Proposals for the preparation of biologically degradable polymer mixtures are known from a large number of patent documents and articles. The great problem in the case of polymer mixtures lies, as a rule, in that those mixtures which have an excellent, biological degradability have only limited possibilities for use in the area of engineering plastics, thereby explaining the relatively modest success to date. Polymer mixtures having improved properties are either biologically inadequate or degradable with increased effort or else are too expensive.
From EP 535 994 a polymer mixture is known consisting essentially of starch and an aliphatic polyester, for example polycaprolactone, where the starch is preferably destructurized with water.
Moreover, in an article in the journal "Starke", Volume 45, No. 9, Sep. 1, 1993, Weinheim, pages 314 to 322, the use of starch in the modification of synthetic plastics is described. Here it is proposed first of all to gelatinize the starch by means of water, in connection with which it is mentioned that this gelatinization takes place by means of an endothermic process. In view of this, one may assume that the "gelatinized starch" mentioned therein is most likely destructurized starch, which is mixed with a range of synthetic plastics in order to develop new composite materials.
In contrast, and in knowledge of the inadequacies of starch destructurized solely with water, it is proposed in DE 42 37 535 for a biologically degradable polymer mixture to incorporate therein thermoplastic starch that is prepared using a suitable plasticizing agent in addition to processing to remove water, either before or during the process of mixing starch with the plasticizing agent. The proposed polymer mixtures contain thermoplastic starch, a hydrophobic polymer and a phase mediator, with a mixture of thermoplastic starch and polycaprolactone being proposed as the preferred mixture.
Aliphatic polyesters are, per se, suitable mixing components for the preparation of biologically degradable polymer mixtures, since they have a good biological degradability. However, aliphatic polyesters have only moderately appropriate material properties, for example, with regard to melting point, tensile strength, and the like, which is why even corresponding mixtures using a polymer prepared on the basis of renewable raw materials, for example thermoplastic starch, have only moderately appropriate properties, thereby again placing in question the possibility of using such blends in the field of engineering plastics.